An in vitro validation of a commercially available metabolic cart using pediatric ventilator volumes

Our objective was to determine the validity of the SensorMedics 2900 Metabolic Measurement Cart (SensorMedics Corp., Yorba Linda, CA) for use at pediatric ventilator volumes by designing an in vitro validation study. The study was performed using a previously described nitrogen dilution, carbon dioxide infusion technique in which known amounts of nitrogen (N2) and carbon dioxide (CO2) are infused while a test lung is ventilated. Metabolic measurements were made at tidal volumes (Vt) varying from 50-500 mL at a peak end-expiratory pressure (PEEP) of 0 and 12 cm H2O and at inspiratory oxygen fractions (Fi,O2s) ranging from 0.21-0.60. Each study was repeated six times to assess reproducibility of measurements. At a PEEP of 0 cm H2O, the measurement of oxygen consumption (V'O2), carbon dioxide production (V'CO2), and minute ventilation (V'E) were all within +/-10% of predicted values at all Fi,O2S except when Vt(t) was 50 mL. When PEEP was increased to 12 cm H2O, error in measurement of V'O2 increased and became unacceptably large as Fi,O2 was increased. Each study consisted of six trials, and results showed acceptable reproducibility as demonstrated by narrow 95% confidence intervals. For V(t)s of 100-500 mL, the metabolic monitor provided accurate and reproducible results under in vitro testing conditions when PEEP was low. Accurate results could not be obtained with a Vt of 50 mL or at elevated PEEP.